Investigation of Rock Mechanics Characteristics and Strength in a Squeezing Tunnel: A Case Study of the Haba Snow Mountain Tunnel

Determining the rock mechanics characteristics and strength is crucial in the effective design of tunnels, particularly in challenging conditions such as squeezing stratum. The focus of this study is on metamorphic basalt, especially its behavior in the Haba Snow Mountain Tunnel Project, which has experienced severe extrusion deformation and collapse of 189m long secondary lining structure. This project is currently the most severely squeezing tunnel in China. Understanding the rock strength in such conditions is of utmost importance. To analyze the rock's properties, thin section identification, XRD diffraction, and scanning electron microscope tests were conducted to determine the mineral composition and microscopic characteristics. Subsequently, the rock's strength was assessed through uniaxial compression, point load, and rebound strength tests. Alongside, the study investigates the mechanical properties of metamorphic basalt and explores the relationship between different rock strength test methods. The findings demonstrate a significant linear correlation between the uniaxial compressive strength, rebound strength, and point load strength of the rocks. The relationship between the point load index and uniaxial compressive strength is represented as y=10.97x, exhibiting a strong correlation coefficient of 0.93. Conversely, the relationship between rebound strength and uniaxial compressive strength is described by y=1.26x-7.39, with a correlation coefficient of 0.76, slightly lower than the point load. Furthermore, as the rock's point load index increases, the confidence region of the relationship between the point load index and uniaxial compressive strength expands. Similarly, the confidence region of the relationship between rebound strength and uniaxial compressive strength initially decreases and then increases with increasing rebound strength. These research findings have significant implications for tunnel design, specifically in understanding the mechanical properties of metamorphic basalt and how different rock strength test methods relate to each other. By considering these relationships, engineers can design tunnels effectively and mitigate potential risks in squeezing stratum conditions.